Add option to pass a file containing the secrets during encoding action

[reynardmh]

Add option to pass secret file

Description

This change lets you pass a file containing the secrets. Copy values.yaml to values.yaml.dec and replace the deliminators with the actual secret, then run:

$ helm vault enc values.yaml -s values.yaml.dec

Type of change

• New feature (non-breaking change which adds functionality)
• This change requires a documentation update

How Has This Been Tested?

$ python3 -m pytest 

Test Configuration:

• Python Version: 3.7.3
• Vault Version: 1.3.0

Tested to work with Helm Version 2.14.3 and 3.0.0

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

[Just-Insane]

These changes look good! Thanks for the tests.